In a conventional heating cooker, an induction heating cooker utilizing an electromagnetic induction is configured to have a casing which accommodates an induction heating coil and a heater which serve as heating sources, as well as an inverter circuit and a control circuit which drive and control these heating sources and the like, and a flat top plate which is provided on the casing and is made of a heat-resistant glass.
A cooking container or the like as an object to be heated and cooked is placed on the top plate in the heating cooker so as to be heated. The cooking container such as a pan is placed at a predetermined position on the top plate, for example, a position which is indicated by a circular mark on the top plate. The induction heating coil and the heater are provided just below the top plate on which the circular mark is indicated. The general heating cooker is configured such that it can simultaneously heat a plurality of cooking containers, for example, three cooking containers, and the circular marks are indicated at the respective heating positions on the top plate.
Operation switches which are operated by a user for an on and off control of a heating operation in the heating cooker, operation switches which are operated by the user for a regulation of a heating calorie and the like, a display lamp and a display mark indicating a used situation of the heating cooker, and the like are provided in a region of a front surface (close to a user) of the top plate, in the light of a usability and a design characteristic. Particularly, as the operation switches which are operated by the user, there has been a configuration in which the electrostatic capacity type touch switch can be operated by touching on the top plate. The electrostatic capacity type touch switch is configured such that a change of an electrostatic capacity of a detecting portion is converted into an operation signal, and is hereinafter abbreviated to an electrostatic touch switch.
As the electrostatic touch switch used in the heating cooker, a configuration thereof has been variously improved. As one of them, there has been an example in which a flat-shaped electrode made of a copper foil is provided on a substrate, and the flat-shaped electrode is brought into close contact with a back surface of the top plate, and is configured as a detecting portion of the electrostatic capacity (for example, refer to Patent Literature 1).
FIG. 16 is a cross sectional view showing an electrostatic touch switch in a conventional heating cooker which is disclosed in Patent Literature 1. In FIG. 16, there is shown a state where a user touches an operation switch region 52 on a surface of a top plate 51 by a finger 53. As shown in FIG. 16, a copper foil flat-shaped electrode 55 formed on an operation circuit board 54 is brought into close contact with a back surface of the top plate 51, and a change of the electrostatic capacity between the flat-shaped electrode 55 and the finger of the user is detected. Accordingly, the flat-shaped electrode 55 serves as the detection portion.
FIG. 17 is an electric circuit diagram showing, as an equivalent circuit, an operation switch serving as an electrostatic touch switch in a conventional heating cooker shown in FIG. 16, and showing a detection circuit for detecting an operation state of the operation switch.
As shown in FIG. 17, the user touches the operation switch region 52 on the surface of the top plate 51 with the finger 53, whereby a capacitor is formed equivalently between the finger 53 and the flat-shaped electrode 55 which is made of the copper foil on the operation circuit board 54, as shown by a broken line in the electric circuit diagram in FIG. 17. Accordingly, the capacitor which is configured by the flat-shaped electrode 55 and the finger 53 through the top plate 51 is charged through a resistance 56 from a power supply. As a result, a non-inverting input terminal (+terminal) of a comparator 57 is changed to a low level (L) from a high level (H) only for a short time. At this time, an output terminal of the comparator 57 changes to the low level (L), an operation signal is output to the control circuit, and on and off of a heating operation of the heating cooker and a control of a regulation of heating calorie are carried out.
As mentioned above, by the operation of the operation switch serving as the electrostatic touch switch, an input voltage to the comparator 57 is changed, and the comparator 57 detects the input voltage so as to output the operation signal to the control circuit. As mentioned above, the comparator 57 is a signal processing portion which processes the signal from the operation switch.
Further, as another electrostatic capacity type touch switch which is used in the conventional heating cooker, there has been an example in which a touch switch is configured by forming a conductive thin film as a detecting portion on a back surface of a top plate (for example, refer to Patent Literature 2).
FIG. 18 is an exploded perspective view which shows in an enlarged manner the periphery of an operation switch region in the conventional heating cooker disclosed in Patent Literature 2. A conductive thin film made of a metal material is attached to a back surface of a top plate 61 shown in FIG. 18. A part of the top plate 61 is provided with a touch operation portion 62 for regulating an output of an induction heating coil. The touch operation portion 62 is provided with an electrostatic capacity type touch switch 64 which has a touch detection electrode 63 configured by a part of the conductive thin film. The touch detection electrode 63 is electrically connected to a connecting line 65 and a conductive electrode terminal 66 which are formed on a back surface of the top plate 61 in accordance with a pattern formation. The conductive electrode terminal 66 is brought into contact with a leaf spring 67 which is made of a conductive material so as to be connected to the control circuit.
Further, in the conventional heating cooker disclosed in Patent Literature 2, it is configured such that a part of the touch switch 64 which is provided just below the top plate 61 shines for improving an operability of the electrostatic capacity type touch switch 64.
FIG. 19 is a plan view showing the touch switch 64 in the operation switch region in the conventional heating cooker which is disclosed in Patent Literature 2. FIG. 20 is a cross sectional view of a substantial part in the touch switch 64 of the conventional heating cooker which is disclosed in Patent Literature 2.
As shown in FIG. 19, the operation switch region in the conventional heating cooker is provided with a contact portion 68 in which a transmission portion 69 transmitting a light therethrough is formed such that a sign or a letter indicating an operation content of the touch switch 64 shines. In the contact portion 68 shown in FIG. 19, there are formed by the transmission portion 69 a display mark indicating the right and left directions for regulating a calorie, and a display mark for operating an on and off control of a heating operation. In the display mark indicating the right and left directions for regulating the calorie, a direction is indicated by a position of an apex of an isosceles triangle, the left touch switch 64 indicates a left direction, and the right touch switch 64 indicates a right direction. Further, in the display mark for operating the on and off control of the heating operation, the left touch switch 64 is an ON operation switch, and the right touch switch 64 is an OFF operation switch.
As shown in FIG. 20, in a main body 71 of the heating cooker, an LED 70 serving as a light emitting light source is arranged just below each of the contact portions 68. An electric conductive coating is printed on the contact portion 68, and the transmission portion 69 is formed such that the light transmits without printing the electric conductive coating. Accordingly, in the contact portion 68 of the touch switch 64, the display mark indicating the operation content is formed by printing the electric conductive coating. The light from the LED 70 which exists just below the contact portion 68 passes through an opening portion of the touch detection electrode 63 and the top plate 61, and irradiates a back surface of the contact portion 68. As a result, it is configured such that the light passes through the transmission portion 69 of the contact portion 68, and the user can recognize the position and the operation content of the contact portion 68.
A plurality of LED 70 respectively corresponding to a plurality of contact portions 68 are each mounted on the circuit board 72. A light shielding portion 73 serving as a partition wall is provided in the periphery of each of the LED 70, so that the light from each of the LED 70 irradiates the contact portion 68 which is arranged thereabove. The light shielding portion 73 serving as the partition wall is fixed onto the circuit board 72, and is extended toward the back surface of the top plate 61. In this case, a top plate side end portion of the light shielding portion 73 is arranged so as to have a gap from the back surface of the top plate.
Further, a leaf spring 74 is provided in the circuit board 72, and the touch detection electrode 63 and the detection circuit of the circuit board 72 are electrically connected through the leaf spring 74.    Patent Literature 1: Japanese Unexamined Patent Publication No. 10-214677    Patent Literature 2: Japanese Unexamined Patent Publication No. 2005-38739